Likely something to do with the hundreds of thousands of people they serve, or something similar. Smaller containers...same prep per plate but more plates....More options in different sizes likely would lead to more mistakes on orders - more food thrown out...Those are reasons I can think of off the top of my head. NOW, one thing I could see is them coming up with an entire CS restaurant dedicated to smaller portions or something - 4 or 5 options, typically an 'adult' menu, but smaller portions. Likely would end up about the same price as the larger ones though, so people would still complain and would likely forego it and just buy the bigger ones for $1.00 more. So I guess it wouldn't work as a lot of people wouldn't opt for same $$ for less food scenario.
There are sides that are smaller, and things can be shared. Buffets are perfect for this - anything you want in any quantitiy you desire. So I suggest lots and lots of buffets.
We tend to do fine by just sharing to get smaller portions...Get an extra side or dessert when needed or wanted. Works out great!